1. Field of the Invention
The invention relates to networks, more particularly to storage area networks (SANs) and yet more particularly to creating snapshots of data on the SAN.
2. Description of the Related Art
Storage Area Networks (SANs) are becoming more common as a means to lower storage costs and increase storage efficiency while increasing storage flexibility. One common function in an SAN is to copy data from one storage device to another, such as during a backup operation. However, performing the backup operation in the SAN is actually relatively difficult, as the SAN is usually running 7/24, so the particular storage units can never be stopped or taken off line to perform the backup. Further, as a SAN often has tera bytes of data, a backup operation is a time consuming process as well. Because the storage devices cannot be taken off line and the backup takes a long time, snapshotting has been developed.
In a snapshot, the data present in the storage unit is captured or snapshotted at the particular instant in time and remains separated from any further data which is written to the storage device. Thus an operation such as backing up can read from the shapshot, while write operations can occur to the particular device, with a write database keeping track of blocks stored to an alternate location to preserve the older data for the snapshot read.
In the past, two different approaches for providing the snapshot functionality have existed. In the first, the functionality is provided in the storage device itself, so that when it receives a snapshot command, it redirects new write data to a new snapshot area on the storage device so that the old data is preserved for snapshot device reads. While this does provide the functionality, it limits the operation to occurring within the particular storage device, thus not allowing spanning across storage devices as a logical unit, as is commonly done in a SAN. The second approach to snapshotting occurred in the host, with host software running to perform the write capture operations which are necessary for snapshot operations. Again, this is a relatively limiting situation in that other hosts could readily access the storage device and thus completely bypass the snapshot operation. So either the software has to be resident on every host, which is problematic by itself, and further complicated by the need for the hosts to communicate, or the storage device must be limited to access only by a particular host computer. Both of these operations are complicated and are contrary to the general development of SANs. Therefore, it is desirable that a much more flexible method be developed to provide snapshot capability in a SAN which is neither storage device nor host limited.